Ostwald nanoemulsions

Wooster, T. J., Golding, M., and Sanguansri, P. (2008). Impact of oil type on nanoemulsion formation and Ostwald ripening stability. Langmuir 24, 12758-12765. 

A poor knowledge of the mechanism of Ostwald ripening, which is perhaps the most serious instability problem with nanoemulsions. 

The only instability problem encountered with nanoemulsions is Ostwald ripening, which is discussed below. 

One of the main problems with nanoemulsions is Ostwald ripening which results from differences in solubility between the small and large droplets. The difference in the chemical potential of dispersed phase droplets between different-sized droplets as given by Lord Kelvin [17],... 

All nanoemulsions showed an increase in droplet size with time, as a result of Ostwald ripening. Figure 14.11 shows plots of versus time for all the nanoemulsions studied. The slope of the Hnes gives the rate of Ostwald ripening CO (in m s ), and this showed an increase from 2 to 39.7x 10 m s as the surfactant concentration was increased from 4% to 8 wl%. This increase may have been due to a number of factors ... 

In contrast to the results obtained with hexadecane, the addition of squalane to the O/W nanoemulsion system based on isohexadecane showed a systematic decrease in Ostwald ripening rate as the squalene content was increased. The results are included in Figure 14.14, which shows plots of versus time for nanoemulsions containing varying amounts of squalane. The addition of squalane up to 20% based on the oil phase showed a systematic reduction in ripening rate (from 8.0 to 4.1 x 10 m s i). It should be noted that when squalane alone was used as the oil phase, the system was very unstable and showed creaming within 1 h. The results also showed that the surfactant used was unsuitable for the emulsification of squalane. 

As expected, the nanoemulsions prepared using high-pressure homogenisation showed a lower Ostwald ripening rate when compared to systems prepared using the PIT method. This is illustrated in Figure 14.18, which shows plots of versus... 

Table 14.4 Ostwald ripening rates for nanoemulsions based on natural oils.

Figure 14.23 shows the results for nanoemulsions based on esters, and the Ostwald ripening rates are listed in Table 14.5. Cj2 i5 alkylbenzoate appeared to give the highest ripening rate. 

Figure 14.24 shows a comparison of two nanoemulsions based on polydecene, a highly insoluble nonpolar oil and PPG-15 stearyl ether which is relatively more polar. Polydecene gave a low Ostwald ripening rate of 6.4 X 10 m s which was one order of magnitude lower than that of PPG-15 stearyl ether (5.5 x 10 m s ). 

Figure 14.25 Influence of glycerol on the Ostwald ripening rate of nanoemulsions.

Unfortunately, the droplet size distribution of a nanoemulsion prepared by the PIT process is relatively large. Due to the high Laplace pressure, Ostwald ripening takes place rapidly, limiting the lifetime of the nanoemulsions to a few minutes to a few days. The addition of a water-insoluble component can significantly reduce the breakdown kinetics however, long-term stability is rarely achieved with this process. 

Unless adequately prepared (to control the droplet size distribution) and stabilized against Ostwald ripening (that occurs when the oil has some finite solubility in the continuous medium), nanoemulsions may show an increasing droplet size and an initially transparent system may become turbid on storage. 

Eigure 1.29 shows the variation of r with time t for 20 80 0/W nanoemulsions at two C12EO4 concentrations prepared by the PIT method. It can be seen from Fig. 1.28 that the emulsion containing the higher surfactant concentration gives a higher rate of Ostwald ripening. This may be due to solubilization of the oil by the surfactant micelles. 

Further evidence for Ostwald ripening was obtained by using a more soluble oil, namely a branched hexadecane (Arlamol HD). The results are shown in Fig. 1.31 for nanoemulsions prepared using 4% surfactant. It can be seen that the more soluble oil (Arlamol HD) give a higher rate of Ostwald ripening when compared with a less soluble oil such as hexadecane. 

The rate of Ostwald ripening is 1.1 x 10 and 2.4 x 10 °m s at 1.6 and 2.4 /o 1NUTEC SP1 respectively. These rates are 3 orders of magnitude lower than those obtained using a nonionic surfactant. Addition of 5 % glycerol was found to decrease the rate of Ostwald ripening in some nanoemulsions. 

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